It is commonplace in the electronic art to combine a modulated or modulating signal with a local oscillator signal in order to obtain a further modulated signal at another frequency that is more easily amplified, filtered, broadcast, and/or detected. This is done in a mixer.
In a typical demodulation application, a modulated radio frequency (RF) signal is combined in a mixer with a local oscillator (LO) signal to produce an intermediate frequency (IF) signal which may be then further amplified and detected to recover the information modulated onto the RF carrier. Alternatively, this process can be reversed, mixing an LO signal with an IF signal to produce a modulated carrier (RF) signal for further amplification and ultimate transmission as a modulated output signal.
The demodulation mixing process produces sum and differences of the RF and LO frequencies. One or more of the sum and difference frequencies is at the desired IF frequency, according to the following relations: EQU f.sub.IF =f.sub.LO -f.sub.RF, i.e., down conversion where f.sub.LO &gt;f.sub.RF, (1) EQU f.sub.IF =f.sub.RF -f.sub.LO, i.e., down conversion where f.sub.LO &lt;f.sub.RF, (2) EQU f.sub.IF =f.sub.LO +f.sub.RF, i.e., up conversion.
Similar relations apply to modulation of a carrier signal.
Examination of equations (1) and (2) shows that there is not a unique correspondence between f.sub.LO, f.sub.IF, and f.sub.RF. For a given value of f.sub.LO, two different values of f.sub.RF may produce the same value of f.sub.IF. For example, (see FIG. 1) for f.sub.LO =3 GigaHertz, both f.sub.RF1 =2.5 GigaHertz and f.sub.RF2 =3.5 GigaHertz can produce f.sub.IF =0.5 GigaHertz. The RF and IF frequencies are generally not discrete frequencies but narrow frequency bands determined by the modulation thereon. The LO frequency is typically sharply defined, but may be time varying in some cases.
A prior art double balanced mixer apparatus 10 is illustrated in FIG. 2. Mixer apparatus 10 comprises input 12, input 42, output 15, balun transformers 16, 40, and four port mixer element 19 comprising diodes 32, 34, 36, 38, and having input ports 24, 26, 28, 30. Signals 14, 18, 20, 44, 46, 48 are present in mixer apparatus 10. RF input signal 14 comprising either or both RF1 and RF2 enters at RF port 12. Balun transformer 16 splits incoming signal 14 into two substantially equal amplitude RF signals 18, 20 which have a relative phase displacement of 180.degree.. Signal 18 is sent to port 24 of four port mixer element 19 and signal 20 is sent to port 28 of mixer element 19. Similarly, LO input 42 supplies LO signal 44 to balun transformer 40. Balun transformer 40 splits LO signal 44 into two substantially equal amplitude RF signals 46, 48 having a 180.degree. relative phase displacement. Signal 46 is sent to port 26 of four port mixer element 19 and signal 48 is sent to port 30 of four port mixer element 19.
The nonlinear current versus voltage characteristics of diodes 32, 34, 36, 38 cause signals to be created at frequencies in accordance with equations 1-3, which signals are coupled to IF port 15. Because balun transformers 16, 40 must be able to pass the RF, LO, and IF frequencies, the required bandwidth of the balun transformers is more difficult to realize. Furthermore, balun transformers such as 16, 40 are generally most useful at frequencies below about one GigaHertz. This limits the frequency range over which prior art mixer apparatus 10 is useful.
Alternatively, mixers are employed for modulation of an LO signal by an IF signal to produce a modulated carrier, or RF signal. This process is similar to the demodulation process described above, with LO port 42 and IF port 15 accepting input signals and RF port 12 providing an output signal.
Prior art mixers have a number of disadvantages well known in the art. Among these disadvantages are, for example: (1) inadequate port-to-port isolation, (2) limited bandwidth, particularly intermediate frequency bandwidth, (3) relative complexity and (4) difficulty of implementation in compact form suitable for incorporation in monolithic microwave integrated circuits (MMIC's).
MMIC's are typically constructed using Si, GaAs, or other compound or elemental semiconductor integrated circuit (IC) wafer processing technology on and/or in such wafers. It is highly desirable to have broadband mixers which can be made with lumped elements or other structures that are compatible with IC fabrication techniques and geometries. In particular, it is important that they be of comparatively small size so as to not occupy disproportionately large substrate areas compared to the semiconductor diodes, transistors, etc., which mix the signals, or compared to the amplifiers or other signal processing elements that may be included in the MMIC. Such concerns are especially important in the frequency range from about 1 to 15 GigaHertz and above where the sizes of distributed circuit elements are unwieldy.
Thus, there continues to be a need for improved broadband mixers and methods that use few components, especially those which are easy to construct and/or which employ elements that are readily integratable in and/or on MMIC's or the like.